1. Field of the Invention
The invention relates generally to the art of processing radar video signals, and more particularly to simultaneously processing video signals from multiple target returns using adaptive threshold settings.
2. Description of the Prior Art
Detection systems for separating targets from noise have been proposed in which the detection threshold is either fixed or automatically adjusted in accordance with variations in background noise and interference. One such prior art system is described in U.S. Pat. No. 4,005,416 issued Jan. 25, 1977 and assigned to the assignee of the present invention. The detection system proposed therein is automatically adjusted in accordance with variations in background noise and interference. The energy levels of video signals within each of three successive range cells are coupled to a comparator wherein the energy contained within a central range cell is compared with the energy contents within the two adjacent range cells. A target indication in a range cell is provided when the ratios of energy in that range cell to the energy in each of the two adjacent cells are both greater than a predetermined value. Noise and interference responses are minimized by summing the output signals of the comparator over a number of range sweeps. A target indication is provided when the sum achieves a specified number before a predetermined number of range sweeps have been completed. The existence of a target requires that both detection criteria be satisfied. This system requires continuous energy comparisons and redundant processing before an existing target is reported. Additionally, the system does not achieve noise reduction in the target area. Targets are reported when the energy in a range cell is greater than the energy in the two adjacent range cells. Thus, the detection threshold rides on the background noise. True noise reduction is not achieved. Further, this detection system does not provide for processing more than one target within a bearing sector.
In a prior art tracking system, disclosed in U.S. Pat. No. 4,070,673, computer generated digital signals representative of leading and trailing edges of range and azimuth intervals defining a tracking window are compared with the instantaneous range and antenna position to provide pulses signifying the opening and closing of a tracking window. This control unit provides signals to a multiplicity of counting circuits wherein the number of pulse transmissions, range bins traversed, and radar target returns within the defined tracking windows are established. The pulse transmission and range bin counters are coupled to accumulators which are enabled by pulses from a comparator, provided when the radar video returns within the tracking window exceed a fixed signal threshold. When the accumulator associated with the pulse transmission counter is enabled, the transmission count, corresponding to the enabling radar return, is added to the total in the accumulator to achieve a weighted sum of angular returns. In a like manner, a radar return enables the accumulator associated with the range bin counter to add the range bin count, associated with the radar return, to the total in the range accumulator. These weighted sums and the radar target return count are utilized to determine the range and angle centroids of a target. Though this system provides accurate tracking of a detected target, it operates at a fixed detection threshold, requiring M of N processing to reduce the probability of false alarms. Further, the system does not have the capability of processing multiple targets on a given bearing.
Other prior art detection and tracking systems use either fixed thresholds for separating targets from noise or sequential methods which establish a predetermined detection or noise threshold and then increment or decrement the threshold in accordance with the observed target magnitudes, and noise and interference levels. Such systems provide less than optimum thresholds in a single scan, or require a multiplicity of scans to achieve an optimum threshold. Further, such prior art systems are incapable of tracking multiple targets on a bearing. Since data for only one target on a bearing can be processed on a single scan, multiple scans are required to update target information for multiple targets on a bearing. Moreover, the prior art systems are subject to switching from tracking one target to tracking a second target (target swapping) if the second target is in the vicinity of the first and its radar return exceeds that of the first.